Technical Field
The invention relates generally to precision measurement instruments, and more particularly to a workpiece camera configuration which may be utilized in an optical pen for chromatic confocal range sensing.
Description of the Related Art
Controlled chromatic aberration techniques may be utilized for distance sensing metrology applications. As described in “Pseudocolor Effects of Longitudinal Chromatic Aberration”, G. Molesini and S. Quercioli, J. Optics (Paris), 1986, Volume 17, No. 6, pages 279-282, controlled longitudinal chromatic aberration (also referred to herein as axial chromatic dispersion) may be introduced in an optical imaging system, causing the imaging system focal length to vary with wavelength, which provides means for optical metrology. In particular, a lens can be designed whose back focal length (BFL) is a monotonic function of wavelength. In white light operation such a lens exhibits a rainbow of axially dispersed foci that can be used as a spectral probe for distance sensing applications.
As a further example, U.S. Pat. No. 7,477,401, which is hereby incorporated herein by reference in its entirety, discloses that an optical element having axial chromatic aberration, also referred to as axial or longitudinal chromatic dispersion, may be used to focus a broadband light source such that the axial distance to the focus varies with the wavelength. Thus, only one wavelength will be precisely focused on a surface, and the axial distance or height of the surface determines which wavelength is best focused. Upon reflection from the surface, the light is refocused onto a small detector aperture, such as a pinhole and/or the end of an optical fiber. Upon reflection from a surface, only the wavelength that is well-focused on the surface is well-focused on the pinhole and/or fiber. All of the other wavelengths are poorly focused on the fiber, and so will not couple much power into the fiber. Therefore, the signal level will be greatest for the wavelength corresponding to the height of the object. A spectrometer at the detector measures the signal level for each wavelength, which effectively indicates the height of the object.
Another configuration for a chromatic confocal range sensor is described in commonly assigned U.S. Pat. No. 7,626,705 (the '705 patent) which is hereby incorporated herein by reference in its entirety.
Certain manufacturers refer to a practical and compact optical assembly that is suitable for chromatic confocal ranging in an industrial setting as a chromatic confocal range sensor and/or as an “optical pen.” One example of optical pen instruments that measure Z height are those manufactured by STIL, S.A. of Aix-en-Provence, France (STIL S.A.). As a specific example, the STIL optical pen model number OP 300NL measures Z heights and has a 300 micron range.
In measurement operations utilizing a chromatic confocal range sensor or optical pen, alignment of a measurement spot of the chromatic confocal range sensor with a particular small region of a workpiece to be measured may be difficult. Known configurations for doing so are cumbersome and/or expensive. In various applications it would be desirable to provide a more convenient, compact, and fast means for alignment of an optical pen measurement spot with a portion of a workpiece.